Correct exception message for XPathResult::snapshotItem.

It previously reported the method called as iterateNext.

This fixes a typo introduced in r162474.
https://codereview.chromium.org/77763005/

BUG=270033

Review URL: https://codereview.chromium.org/80943002

git-svn-id: svn://svn.chromium.org/blink/trunk@162479 bbb929c8-8fbe-4397-9dbb-9b2b20218538